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Role of SO2 for Elemental Mercury Removal from Coal Combustion Flue Gas by Activated Carbon Md. Azhar Uddin,* Toru Yamada, Ryota Ochiai, and Eiji Sasaoka Department of Material and Energy Science, Graduate School of EnVironmental Science, Okayama UniVersity, Tsushima Naka, Okayama 700-8530, Japan
Shengji Wu Department of EnVironmental Science and Technology, School of Mechanical Engineering, Hangzhou Dianzi UniVersity, Xiasha Higher Education Zone, Hangzhou 310018 China 8530 ReceiVed February 22, 2008. ReVised Manuscript ReceiVed April 18, 2008
In order to clarify the role of SO2 in the removal of mercury from coal combustion flue gas by activated carbon, the removal of Hg0 vapor from simulated coal combustion flue gas containing SO2 by a commercial activated carbon (AC) was studied. The Hg0 removal experiments were carried out in a conventional flow type packed bed reactor system with simulated flue gases having a composition of Hg0 (4.9 ppb), SO2 (0 or 500 ppm), CO2 (10%), H2O (0 or 15%), O2 (0 or 5%), and N2 (balance gas) at a space velocity (SV) of 6.0 × 104 h-1 in a temperature rang 60-100 °C. It was found that, for SO2 containing flue gas, the presence of both O2 and H2O was necessary for the removal of Hg0 and the Hg0 removal was favored by lowering the reaction temperature in the order of 60 > 80 > 100 °C. The presence of SO2 in the flue was essential for the removal of Hg0 by untreated activated carbon. The activated carbons pretreated with SO2 or H2SO4 prior to the Hg0 removal also showed Hg0 removal activities even in the absence of SO2; however, the presence of SO2 also suppressed the Hg0 removal of the SO2-pretreated AC or H2SO4 preadded AC.
Coal combustion power plants and municipal waste incineration plants are major anthropogenic sources of mercury emissions. Mercury is emitted in flue gases mainly in an elemental (Hg0) or oxidized state (HgCl2 or HgO). Hg is also emitted as particulate bound Hg (Hgp) during coal combustion; however, the Hgp is attached to the fly ash and captured in the electrostatic precipitator (ESP).1 Elemental mercury is more difficult to remove from the flue gas stream due to its low melting point, high equilibrium pressure and low solubility in water. Hg vapor is not effectively captured in typical air-pollution control devices. Air-pollution control devices like wet flue gas desulfiziation (WFGD) system can ca. 90% of the oxidized mercury, but can not remove any Hg0.2,3 Adsorption of mercury vapor using activated carbon impregnated with sulfur, chlorine and iodine is currently the most widely use technology for mercury removal from incineration flue gas.4–9 However, the major drawbacks of activated carbons
are high cost, poor capacity, narrow temperature range, and slow regeneration and adsorption rates. Hence, development of a novel method using low-cost adsorbents as an alternative to sulfur-impregnated activated carbon is of great importance. Our research group aimed to develop a process which can remove, condense, and recover mercury compounds from flue gases. Recently, we have developed a novel method using solid adsorbents for effective removal of Hg0 vapor from simulated flue gas containing H2S.10–11 This method is based on the reaction of H2S and Hg on the adsorbents. We have also reported that Hg0 can be removed from the simulated flue gas by activated carbon in the presence of SO2 and absence of H2S.10 However, the reaction mechanism and the role played by SO2 in the removal of Hg0 vapor in the absence of H2S are not well understood yet. Flue gas composition has significant effects on the adsorption of Hg0 on carbon-based sorbents.12–17 Miller et al. have conducted the mercury breakthrough test with simulated flue gas on lignite-based activated carbon (LAC) in a laboratory scale
* Corresponding author. Fax: +81-86-251-8897. E-mail: alazhar@ cc.okayama-u.ac.jp. (1) (a) Galbreath, K. C.; Zygarlicke, C. EnViron. Sci. Technol. 1996, 30, 2421–2426. (b) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Fuel Process. Technol. 2003, 82, 89–165. (2) Zhuang, Y.; Zygarlicke, C. J.; Galbreath, K. C.; Thompson, J. S.; Holmes, M. J.; Pavlish, J. H. Fuel Process. Technol. 2004, 85, 463–472. (3) Sinha, R. K.; Walker, P. L. Carbon 1972, 10, 754–6. (4) Otani, Y.; Kanaoka, C.; Usui, C.; Matuui, S.; Emi, H. EnViron. Sci. Technol. 1986, 20, 735. (5) Krishnan, S. V.; Bullett, B. K.; Jozewicz, W. EnViron. Sci. Technol. 1994, 28, 1506–12. (6) His, H. C.; Rostam-Abadi, M. J.; Chen, M.; Chang, S. EnViron. Sci. Technol. 2001, 35, 2785–91.
(7) Lee, S. J.; Seo, Y-C,; Jurng, J.; Lee, T. G. Atmos. EnViron. 2004, 38, 4887–4893. (8) Nakajima, W.; Tougaki, N.; Wu, S.; Nagamine, S.; Sasaoka, E. Removal of Gas Phase Mercury by Solid Sorbent. Trace Element Workshop, Yokohama, July 18-19, 2002. (9) Togaki, N.; Uddin, M. A.; Nakasima, W.; Nagamine, S.; Sasaoka, E. Activity of Adsorbents for Removal of Mercury Vapor with H2S Proceedings of the 20th Annual International Pittsburgh Coal Conference; Pittsburgh, PA, September 19-15, 2003. (10) Morimoto, T.; Wu, S.; Uddin, M. A.; Sasaoka, E. Fuel 2005, 84, 1968–74. (11) Wu, S.; Morimoto, T. ; Togaki; N.; Nagamine, S.; Uddin, M. A. ; Sasaoka; E. Characteristics of Activated Carbon for Hg Removal of Flue Gas with H2S and Iron Oxide for Hg Removal of Coal derived Fuel Gas with H2S. 227th ACS National Meeting; Anaheim, CA, April 1, 2004.
1. Introduction
10.1021/ef800134t CCC: $40.75 2008 American Chemical Society Published on Web 06/11/2008
Mercury RemoVal from Coal Combustion Flue Gas
reactor at about 100 °C. When the sorbent was exposed to SO2 (mixture of O2, CO2, N2, and H2O) in addition to baseline gas, LAC sorbent capture improved slightly. Upon exposure of the sorbent to the HCl, NO, or NO2 added one at a time to the baseline gases, the mercury capture improved to 90-100%. However, the interaction between SO2 and NO2 caused rapid decrease in mercury capture capacity of the sorbent.12 Recently, Presto et al. have reported that mercury capture with activated carbon injection was suppressed in flue gases containing high concentrations of sulfur oxides (SOx). The final mercury content of the activated carbons was independent of the SO2 concentration in the SFG, but the presence of SO3 inhibits mercury capture even at the lowest concentration tested (20 ppm).,14 Bench-scale testing of Hg0 sorption on selected activated carbon sorbents was conducted to develop a better understanding of the interaction among the sorbent, flue gas constituents, and Hg0 by Olson et al.15 They explained the role of acid gases in simulated flue gas on mercury capture with XPS of the sorbents: a competition between the bound hydrogen chloride (HCl) and increasing sulfur [S(VI)] for a basic carbon binding site affected mercury removal performance of the sorbents. Huggins et al. studied the mercury adsorbed species on various carbonaceous sorbent materials. Their data from S and Cl XANES spectra, as well as from the Hg XAFS data, strongly support the hypothesis that interaction of acidic species (HCl, HNO3, H2SO4, etc.) in the flue gas with the sorbent surface is an important mechanistic process that is responsible for creation of active sites for mercury capture by chemisorption.16 Although there have been many studies about the importance of the presence of SO2 in the removal of Hg from flue gas. However, the role played by SO2 in Hg0 removal is complicated by the presence of other acidic gases such as NOx and HCl as mentioned above. In this study, we made an attempt to clarify the role of SO2 in Hg0 removal by activated carbon in more simplified conditions, i.e., in the absence of NOx and HCl. The main purpose of this study is to clarify the mechanism of mercury removal in the presence of SO2 in the flue gas. In this study, commercial activated carbon was used as uniformity of the sample is necessary to study the characteristics Hg0 removal reactions. 2. Experimental Section 2.1. Activated Carbon Sample. Activated carbon produced from coconut shell (AC) was purchased from Wako Pure Chemical Co. Ltd. The granular AC particles were crushed and sieved into an average diameter of 1.0 mm. The surfaced area of the sample was measured by a conventional N2 adsorption method (Micromeritics Gemini 2375). The coconut shell AC had a specific surface area of 1250 m2/g and a surface area per unit packed volume of 472 m2/cc. In one instance, activated carbon prepared from heavy oil fly ash and asphalt was also used. The details of the preparation (12) Miller, S. J.; Dunham, G. E.; Olson, E. S.; Brown, T. D. Fuel Process. Technol. 2000, 65-66, 343–363. (13) Presto, A. A.; Granite, E. J.; Karash, A. Ind. Eng. Chem. Res. 2007, 46, 8273–8276. (14) Presto, A. A.; Granite, E. J. EnViron. Sci. Technol. 2007, 41, 6579– 6584. (15) Olson, E. S.; Crocker, C. R.; Benson, S. A.; Pavlish, J. H.; Holmes, M. J. J. Air Waste Manage. Assoc. 2005, 55, 747. (16) Huggins, F. E.; Yapa, N.; Gerald, P.; Huffman, G. P.; Senior, C. L. Fuel Process. Technol. 2003, 82167–196. (17) Li, Y. H.; Serre, S. D.; Lee, C. W.; Gullett, B. K. Elemental Mercury Adsorption by Activated Carbon Treated with Sulfuric Acid. Proceedings of the 2001 Mega Symposium/Mercury Emissions: Fate, Effects & Control, Chicago, IL, August 20-23, 2001.
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Figure 1. Effect of temperature on the Hg0 removal by AC: (feed gas) Hg0 (4.9 ppb), SO2 (500 ppm), CO2 (10%), H2O (14.7%), O2 (5%), and N2 (balance gas) at 500 cm3 STP/min (SV: 6.0 × 104 h-1).
of fly ash AC were given elsewhere.18 Fly ash AC has a BET surface area of 72 m2/g. 2.2. Apparatus and Procedure. Mercury vapor (Hg0) removal performance of the activated carbon adsorbent was carried out in a fixed-bed flow type reactor. A detailed description of the experimental setup and procedure is given elsewhere.12 The test rig consisted of a mercury permeation device; a simulated coal combustion flue gas feed system, a tubular reactor (size:13.5 mm) with an electric furnace; and a mercury analysis system. The mercury permeation device, designed to deliver controlled amount of Hg0 vapor, is an array of liquid mercury containing u-tubes connected in series immersed in an ice-water bath. A controlled amount of nitrogen gas is introduced into the inlet of the horizontally laid mercury containing u-tubes which carry trace amounts of elemental vapor and the mercury concentration can be varied by varying the nitrogen flow rate. The model coal combustion flue gases were simulated by mixing the stream of N2 carrying Hg0 vapor and other gas streams before entering the reactor. All lines of feed gases after H2O injection were heated to 100 °C with a tape-heater. Prior to the Hg0 removal test run, about 0.5 mL of the activated carbon sample (size: 1 mm) was packed into the quartz tube reactor The evaluation of the reactivity of the sample was performed at atmospheric pressure in a temperature range of 60 to 100 °C. The reaction of Hg removal reaction commenced when a mixture of Hg0 (4.9 ppb), SO2 (0 or 500 ppm), CO2 (10%), H2O (14.7%), O2 (5%), and N2 (balance gas) was fed into the reactor at 500 cm3 STP/min (space velocity (SV): 6.0 × 104 h-1). These conditions will be referred as the “standard reaction system” hereafter. Measurement of the inlet and outlet concentration of mercury was carried out using a cold vapor mercury analyzer. The mercury adsorption efficiency was quantified by comparing the Hg0 contents of the gas before and after adsorption.
3. Results and Discussions 3.1. Effect of the Temperature on Hg0 Removal by Activated Carbon. Effect of the temperature on the removal of Hg0 by AC was examined in a temperature range of 60 to 100 °C. The Hg0 removal was favored at lower temperature in the order 60 > 80 > 100 °C as shown in Figure 1. The dependency of the removal rate of mercury on the temperature is reasonable if the mercury is removed by physical adsorption; however, the dependency is not explained if the mercury is chemically fixed by the AC. Effect of temperature on Hg0 removal efficiency is discussed further later in section 3.6. We conducted the Hg removal experiments mainly at 80 °C with a target that Hg could be removed by activated carbon from the combustion flue gas before the wet scrubber. Desulfurization efficiency in the wet (18) Uddin, M. A.; Shinozaki, Y.; Furusawa, N.; Yamada, T.; Yamaji, Y.; Sasaoka, E. J. Anal. Appl. Pyrolysis 2007, 78, 337–342.
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Figure 3. Effect of the addition of H2SO4 on the Hg0 removal by AC: (feed gas) Hg0 (4.9 ppb), SO2 (0 or 500 ppm), CO2 (10%), H2O (15%), O2 (5%), and N2 (balance gas) at 500 cm3 STP/min (SV: 6.0 × 104 h-1); (temperature) 80 °C.
Figure 2. Effect of the presence of SO2, O2, and H2O on the Hg0 removal by (a) coconut shell AC and (b) fly ash AC: (feed gas) Hg0 (4.9 ppb), SO2 (0 ppm), CO2 (10%), H2O (0 or 15%), O2 (0 or 5%), and N2 (balance gas) at 500 cm3 STP/min (SV: 6.0 × 104 h-1); (temperature) 80 °C.
scrubber is high at 40-60 °C. Therefore, we selected 80 °C as a test condition for Hg removal by activated carbon. 3.2. Effect of the Presence of H2O, O2, and SO2 on Hg0 Removal by AC. The effect of the presence of H2O, O2, and SO2 on the Hg0 removal by AC was examined at 80 °C. As shown in Figure 2, it was confirmed that the presences of SO2 and O2 are essential for Hg removal. Furthermore, the presence of H2O drastically accelerates Hg0 removal. From these results, we suggest that the following reactions contribute to the Hg0 removal as intermediate steps. SO2+1/2O2 ) SO3
(1)
SO3+H2O ) H2SO4
(2)
Figure 2b shows the effect of the presence of SO2 in the gas stream on the Hg0 removal for the fly ash AC. A similar effect of SO2 was observed, i.e., the presences of SO2 was necessary for the removal of Hg0 by fly ash AC. 3.3. Effect of the Addition of H2SO4 to AC on Hg0 Removal. As it was assumed that H2SO4 formed from SO2, O2, and H2O contributed to the Hg0 removal, the effect of the addition of H2SO4 to AC was examined. The amount of H2SO4 to be added to the AC was determined by assuming that 500 ppm of SO2 is flowed through the reactor over the AC for 4 h and 10% of that SO2 i.e., 100 ppm of SO2 is converted to H2SO4 on the AC. About 1 mL of AC sample particles were soaked with 15 mL of 0.07N H2SO4-aq in a flask and then the extraneous H2SO4-aq was vaporized at 50-60 °C using a conventional rotary evaporator. The sample was dried at 110 °C for 25 h. As shown in Figure 3, it was confirmed that the added H2SO4 enhanced the Hg0 removal performance of the AC
Figure 4. Effect of the treatment of AC with SO2 on the Hg0 removal: (feed gas) Hg0 (0 and 4.9 ppb), SO2 (0 or 500 ppm), CO2 (10%), H2O (15%), O2 (5%), and N2 (balance gas) at 500 cm3 STP/min (SV: 6.0 × 104 h-1); (temperature) 80 °C.
sample. From this result it was thought that if reactions 1 and 2 occur over the AC, the deposited H2SO4 can contribute to the Hg0 removal. Therefore, the effect of treatment with SO2, O2, and H2O prior the removal of Hg0 was examined. As shown in Figure 4, the SO2-pretreated sample was also active for Hg0 removal and the Hg removal efficiency of the sample was a little lower than that of the H2SO4-added sample. From these results, the contribution of the reactions of eqs 1 and 2 to the Hg0 removal was confirmed. However, the removal rate of Hg0 in the copresence of SO2, O2, and H2O was considerably lower than that of the SO2-pretreated sample in the absence of SO2. This result seems unreasonable if reactions 1 and 2 occurred during the Hg0 removal in the copresence of SO2, O2, and H2O in a similar manner as in the SO2-pretreatment. Li et al. have reported that a H2SO4-treated activated carbon revealed a very Hg0 capture capacity compared to the nontreated activated carbon.18 It was suggested that Hg0 adsorption capacity of H2SO4-treated activated carbon can be explained by the enhanced adsorption potential and enthalpy of adsorption and the enhancement be resulted from the narrow microporosity and increased surface polarity of the carbon due to H2SO4 impregnation. Presto et al. have reported that mercury capture with activated carbon injection was suppressed in flue gases containing high concentrations of sulfur oxides (SOx). The final mercury
Mercury RemoVal from Coal Combustion Flue Gas
Figure 5. Effect of the presence of SO2 on the Hg0 removal by the SO2-pretreated AC: (feed gas) Hg0 (0 and 4.9 ppb), SO2 (0 or 500 ppm), CO2 (10%), H2O (15%), O2 (5%), and N2 (balance gas) at 500 cm3 STP/min (SV: 6.0 × 104 h-1); (temperature) 80 °C.
content of the activated carbons was independent of the SO2 concentration in the SFG, but the presence of SO3 inhibits mercury capture even at the lowest concentration tested (20 ppm). The mercury removal capacity decreased as the sulfur content of the used activated carbons increased from 1 to 10%. Their results suggest that mercury and sulfur oxides are in competition for the same binding sites on the carbon surface.14,15 In contrast to the above findings, our present study results revealed that both the SO2- and H2SO4-treated activated carbon enhanced the Hg0 capture capacity. In our study, the amount of H2SO4 loaded on the AC was about 0.04 wt % as sulfur, which is an order of magnitude smaller than the above case. Therefore, the physiochemical states of the adsorbent and the environment of mercury adsorption environment (such as, absence of HCl) in our study are quite deferent from that of the above-mentioned study. 3.4. Effect of the Presence SO2 on Hg0 Removal by the SO2-Pretreated AC. From the above-mentioned results, it is evident that the presence of SO2 suppressed the Hg0 removal in the SO2-pretreated sample. Therefore, the effect of the presence of SO2 on the Hg0 removal over the SO2 treated sample was examined. During the mercury removal experiment with the SO2-pretreated sample, SO2 was intermittently fed to the reaction system. As shown in Figure 5, the presence of SO2 suppressed the mercury removal, but the mercury removal efficiency of the sample recovered after the SO2 feed was stopped. If some H2SO4 was formed on the sample during the SO2 pretreatment, the effect of the presence of SO2 with the pretreated sample should be observed over the H2SO4 added sample. Therefore, the effect of the presence of SO2 on the mercury removal by the H2SO4 added sample was examined. As shown in Figure 6, it was confirmed that the presence of SO2 suppressed the mercury removal by the H2SO4 added sample. Furthermore, the value of the fractional removal of mercury was almost same as that of nontreated sample in the standard reaction system [Hg0 (4.9 ppb), SO2 (500 ppm), CO2 (10%), H2O (14.7%), O2 (5%), and N2]. This result suggest that the removal rate of the mercury by the nontreated sample is not directly correlated with the formation rate of H2SO4 with the sample: the rate determining step of mercury removal is not the formation of H2SO4. If this hypothesis is reasonable, the direct reaction of mercury with H2SO4 is not the rate the determining step for mercury removal. Therefore a stepwise reaction can be assumed: the mercury reacts with oxygen to form a stable mercury compound. HgSO4 is one of the stable
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Figure 6. Effect of the presence of SO2 on the Hg0 removal by the H2SO4 added AC: (feed gas) Hg0 (4.9 ppb), SO2 (0 or 500 ppm), CO2 (10%), H2O (15%), O2 (5%), and N2 (balance gas) at 500 cm3 STP/ min (SV: 6.0 × 104 h-1); (temperature) 80 °C.
Figure 7. Effect of the presence of O2 on the Hg0 removal by the SO2pretreated AC: (feed gas) Hg0 (0 or 4.9 ppb), SO2 (0 or 500 ppm), CO2 (10%), H2O (15%), O2 (0 or 5%), and N2 (balance gas) at 500 cm3 STP/min (SV: 6.0 × 104 h-1); (temperature) 80 °C.
compounds. If HgSO4 is produced in this system, the following reactions can be assumed to occur: Hg + 1/2O2 ) HgO
(3)
HgO + H2SO4 ) HgSO4 + H2O
(4)
The gas phase oxidation of Hg by O2 begins at over 300 °C, however oxidation of Hg can also occur at an appreciable rate below 300 °C in the presence of activated carbon which acts as a catalyst.19 3.5. Effect of the Presence O2 on Hg0 Removal by the SO2-Pretreated AC in the Absence of SO2. The abovementioned results suggest that the role of oxygen is important. In order to confirm the contribution of oxygen, the effect of the presence O2 on the Hg0 removal by the SO2 treated AC in the absence of SO2 was examined. As shown in Figure 7, it was confirmed that the presence of O2 accelerated the Hg0 removal. However, the Hg0 removal efficiency remained high even in the absence of O2 in the feed gas. It was thought that small amount oxygen remained still in this system after displacement with oxygen free gases. If some parts per million of the oxygen remained in the reaction system, the value is very large in comparison with an Hg concentration of 5 ppb. It is thought that the remaining oxygen could contribute to the reaction. The effect of the presence of oxygen on the H2SO4 added sample was also examined. As shown in Figure 8, it was (19) Hall, B.; Schager, P.; Lindqvist, O. Water, Air Soil Pollut. 1991, 56, 3–14.
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0
Figure 8. Effect of the presence of O2 on the Hg removal by the H2SO4-added AC: (feed gas) Hg0 (4.9 ppb), CO2 (10%), H2O (15%), O2 (0 or 5%), and N2 (balance gas) at 500 cm3 STP/min (SV: 6.0 × 104 h-1); (temperature) 80 °C.
Figure 9. Mechanism of mercury removal by AC in the presence of SO2, O2, and H2O.
confirmed that in the absence of oxygen the fractional removal of mercury decreased with lapsed time and when oxygen was added, the reactivity was recovered. The results observed in these two experiments can be explained if we assume that reaction 3 contributes to the mercury removal. 3.6. Mechanism of Hg0 Removal in the Presence of SO2 by AC. From the experimental results in this study, we present a schematic diagram of the reaction route. As shown in Figure 9, SO2 reacts with O2 (route 1) and then forms H2SO4 (route 2). Hg reacts with oxygen and produces HgO (route 3). The HgO then reacts with H2SO4 and produces a SO4 containing complex. This complex contain HgSO4; however, it is very difficult to confirm the formation of H2SO4: the amount of the complex is too small to analyze because the inlet concentration of Hg is only about 5 ppb(v). The formation of the complex from the direct reaction of Hg with H2SO4 could also not be confirmed, because an oxygen free reaction condition could not be obtained using our reactor. The suppression of mercury removal by the presence of SO2 be explained if routes 5 and 6 occur. Therefore, the reactions of routes 5 and 6 were examined. After the 2 h of Hg removal experiment with the SO2-pretreated AC (SV: 12 × 104 h-1), the used sample was treated with SO2(500 ppm) containing gases at 80 °C. As shown in Figure 10, the evolution of Hg was observed in the first stage of the SO2 treatment. The amount of Hg evolved was about 22% of the removed Hg. As the removed Hg was not desorbed in the absence of SO2 gases, it was thought that the Hg evolution was induced by the reduction of a part of the Hg complexes over the AC sample with SO2. If
Figure 10. Evolution of Hg from of the used SO2-pretreated AC for the Hg0 removal with the SO2 containing reactant gases in the absence of Hg: (SO2 pretreatment conditions) CO2 (10%), H2O (15%), O2 (0 or 5%), SO2 (500 ppm), and N2 (balance gas), 1 h, at 80 °C; (Hg removal experiment conditions) Hg0 (4.9 ppb), CO2 (10%), H2O (15%), O2 (0 or 5%), and N2 (balance gas), at 500 cm3 STP/min (SV: 6.0 × 104 h-1) for 2 h, at 80 °C. Reaction systems (1) bypass: N2, CO2, O2, H2O, SO2; (2) reactor: N2, CO2, O2, H2O, SO2; (3) bypass: N2, CO2, O2, H2O, SO2, Hg.
Figure 11. Evolution of Hg0 from of reagent grade HgO with the SO2 containing reactant gases without Hg in the feed gas at 80 °C: (inlet gases) CO2 (10%), H2O (15%), O2 (5%), SO2 (0 or 500 ppm), and N2 (balance gas). Reaction systems (1) reactor: N2, CO2, O2, H2O; (2) bypass: N2, CO2, O2, H2O; (3) bypass: N2, CO2, O2, H2O, SO2; (4) reactor: N2, CO2, O2, H2O, SO2; (5) bypass: N2, CO2, O2, H2O, SO2, Hg. Feed gas flow rate 500 cm3 STP/min (SV: 6.0 × 104 h-1). Temperature 80 °C.
the scheme shown in Figure 9 is correct, the surface Hg compounds are composed of HgO and Hg2+/SO4 complex (possibly containing HgSO4). Therefore, it was assumed that the reduced Hg compound was evolved from the reduction of HgO by SO2 and the remainder is Hg2+/SO4 complex. The reducibility of HgO by SO2 was examined using a reagent grade HgO powder. This reagent was purchased from Wako Pure Chemical Co. Ltd. A 20 mg of the HgO powder (as received) was packed in the reactor and a gas mixture containing Hg0 (0 or 4.9 ppb), SO2 (0 or 500 ppm), CO2 (20%), O2 (5%), H2O (16%), and N2 was fed into the reactor at 500 cm3 STP/min. As shown Figure 11, a considerable amount of Hg was formed by the contact of HgO and SO2 at 80 °C. Therefore, it was thought that the following reaction occurred between HgO and SO3: HgO + SO2 ) Hg0 + SO3
(5)
Reaction 5 can be affected by the presence of O2, because reaction 1 occurs. Therefore, the effect of the absence of O2 on the Hg0 evolution from HgO was examined. In the absence of O2, the evolution of Hg by the SO2 treatment was also observed. From this result, it is suggested that reaction 5 might occur predominantly in comparison with reaction 1. The occurrence
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of reaction 5 in the SO2-N2 system was also directly confirmed. However, the evolution of Hg was not observed. Why the presence of H2O was necessary for the evolution of Hg from HgO reduction with SO2 is unknown at this stage. Possibly the following reaction occurs during Hg evolution. HgO + H2O + SO2 ) Hg + H2SO4
(6)
Hg0
The fractional determination of HgO and is controlled by the oxidation of Hg0 by O2 to HgO (reaction 3) and reduction of HgO to Hg0 by SO2 as shown in reaction 5. The removal rate of Hg0 by oxidation decreases with the increase of temperature (as mentioned in Figure 1) because the reduction of HgO with SO2 occurred preferably compared to Hg0 oxidation.
The Hg0 removal was favored at lower temperatures in the order of 60 > 80 > 100 °C. The presence of SO2 in the flue gas was essential for the removal of Hg0 vapor by untreated activated carbon. However, the activated carbons pretreated with SO2 or H2SO4 prior to the Hg0 removal also showed Hg0 removal activities even in the absence of SO2. For SO2 containing flue gas, the presence of both O2 and H2O was necessary for the removal of Hg0 vapor. However, the presence of SO2 suppressed the Hg removal activities of both H2SO4-preadsorbed AC and SO2pretreated AC. It was reveal that SO2 was indispensable for Hg removal by AC, but also suppressed the Hg removal. It is suggested that the suppression of Hg removal efficiency of AC by SO2 was induced by the reduction of HgO to Hg by the SO2 in the presence of H2O. We have presented a reaction scheme that could explain the experimental results.
4. Conclusions In this work, we studied the effects of gas composition, particularly of SO2, O2, and H2O in the simulated coal combustion flue gas on the Hg0 removal performance of commercial granular activated carbons (AC) derived from coconut shell (commercial).
Acknowledgment. This work was partly supported by the Grantin-Aid for Scientific Research on Priority Areas (B) from Ministry of Education, Science, Sports and Culture, Japan (No. 18310056). EF800134T
